With the continuing trend of increased device densities in integrated circuits (ICs), such as deep submicron technologies, manufacturers are required to achieve equal or better performance. One baseline parameter that is utilized in most integrated circuits is a reference voltage. In circuits having higher performance requirements, a temperature stable reference voltage is required. One type of reference voltage circuit is known as a bandgap reference circuit. The main principal of generating a bandgap reference voltage is to balance a negative temperature coefficient of a pn junction with a positive temperature coefficient of thermal voltage (Vt=kT/q), where k is Bolzman's constant, T is absolute temperature (Kelvin) and q is charge.
A typical bandgap circuit relies on two groups of transistors running at different emitter current densities. A rich transistor, for instance, will typically operate at a current density that is much (e.g., about ten times) greater than the current density of a leaner transistor. The difference in current densities will further cause a difference between the base-emitter voltages of the two transistor groups, referred to as a delta base-emitter voltages (ΔVbe). The ΔVbe is usually amplified by a factor (e.g., about 10) and added to a Vbe voltage of a transistor. The sum of these two voltages adds up to about 1.2 volts, which is approximately the bandgap of silicon.
In many integrated circuits where high performance is required, the bandgap reference can be trimmed to a desired target voltage. For instance, the bandgap reference voltage may be required to provide a voltage within a predefined range of a desired voltage, which can be specified in terms of a percentage of variation of the bandgap reference. The specified voltage can vary according to the application of the IC. Where a precision bandgap reference is not required for a particular IC, the bandgap voltage can remain untrimmed.
Since bandgap reference voltages may play a pivotal role in establishing the accuracy and performance of many integrated circuits and the systems in which they are implemented, various trimming techniques and algorithms have been developed to compensate for process variations, temperature, and complex second-order and third-order effects. The trimming process typically is performed during late stages of IC fabrication and includes a scan of the trim codes to meet a desired target bandgap reference voltage. As a common example, the trimming can be performed on individual die on the wafer.
As device dimensions shrink into smaller submicron sizes, improved trimming procedures are needed to help ensure adequate performance of ICs for their intended applications.